I don't think going 2 pounds over on one side is going to be that bad. The stroller isn't going to break and cause him to fall on the sidewalk or anything.
But I'm guessing since you don't mention having a double stroller of your own that he usually walks, right? Can he walk when you spend the day at the zoo or something else with lots of walking all day long? If so, what I would do (and what I have done in your situation), is just take a good single stroller plus a baby carrier or sling of some sort. I love my Ergo. You can have your 4 year old walk for most of the day, but when he gets too tired to go on (in the evenings and maybe right before naptime), you can wear the baby and let the older one ride.
You'll probably want to wear the baby at other times, too, since it's the easiest way to carry a baby in lines and on rides. In our family, the 4 year old would continue to walk when the baby was in the Ergo most of the time so that the stroller would be available to the baby as soon as he wanted it without any whining from a displaced 4 year old. However, when the 4 year old was truly tired, I'd wear the baby and let the older one ride.